"We currently have a large position in APPLE. We believe the company to be extremely undervalued. Spoke to Tim Cook today. More to come," tweeted Icahn on Tuesday afternoon, US time.

He posted a follow-up tweet four minutes later, saying he would press Apple to increase its stock buyback, in which the company purchases its own shares in a bid to boost its value.

Advertisement

"Had a nice conversation with Tim Cook today. Discussed my opinion that a larger buyback should be done by now. We plan to speak again shortly."

The markets responded almost immediately to the tweets, which totalled just 49 words, with trading volume ballooning from about 200,000 shares changing hands to more than 829,000 shares. Apple shares jumped more than 5 per cent to $US494.66 – the highest since January.

The shares, which had been flat for most of the day before Icahn's tweets, later closed 4.75 per cent higher at $US489.57.

"We appreciate the interest and investment of all our shareholders," said Apple spokesman Steve Dowling. "Tim had a very positive conversation with Mr Icahn today."

We currently have a large position in APPLE. We believe the company to be extremely undervalued. Spoke to Tim Cook today. More to come.

An anonymous source told the Wall Street Journal that Icahn has about a $US1 billion stake in Apple. If this is correct, Icahn would have made about $US50 million just from the surge caused by his tweets.

His investment firm Icahn Enterprises released a statement saying Icahn will use Twitter to "communicate with the public about our company and other issues".

Shares in the California tech giant surged to an all-time high of $US705.05 in September last year, but have since fallen more than 30 per cent with many analysts concerned the company has lost its innovative edge.

Despite this, Apple remains the most valuable publicly traded company with a market capitalisation of more than $US440 billion.

Had a nice conversation with Tim Cook today. Discussed my opinion that a larger buyback should be done now. We plan to speak again shortly.

In April, Apple pledged to spend $US60 billion buying back its stock until 2015 as a way to return cash to shareholders. About $US18 billion of that commitment already had been spent as of June 29, according to the company's regulatory filings. Apple also plans to pay out more than $US10 billion in shareholder dividends each year.

Icahn, whose wealth is estimated by Forbes magazine to be $US20 billion, said Apple "has the ability to do a $US150 billion buyback now by borrowing funds at 3 per cent".

"If Apple does this now and earnings increase at only 10 per cent, the stock – even keeping the same multiple currently – should trade at $US700 a share."

Apple shares earlier posted gains on Monday, following reports the company planned to release a new iPhone model at a September 10 event, and new iPad models later in the year.

Icahn, who has a history of taking stakes in firms to seek shake-ups, is currently in a battle with Dell as he tries to stop the ailing PC maker from selling itself to founder and chief executive Michael Dell. Icahn has previously clashed with management at Yahoo!, Time Warner and BlockBuster.